The preparation of polyisocyanates containing isocyanurate groups has been known for a long time and is described in a large number of publications and patents (Houben-Weyl, Methoden der organischen Chemie Volume 8, p. 136 ff., Georg Thieme verlag Stuttgart 1952; H. Wagner, H. F. Sarx, Lackkunstharze 5th Edition, page 153 ff., Carl Hanser Verlag Munich 1971; DE-A 4 428 107, U.S. Pat. No. 2,993,870; DE-C 1 201 992; DE-A 2 452 532; J. prakt. Chem. 336, pp. 185 to 200, 1994). Both trimers based on aliphatic and trimers based on aromatic diisocyanates are employed universally as paint base materials and also as polyurethane elastomers and polyurethane foams.
From an occupational hygiene standpoint low-monomer-content trimer product grades are preferred. These products are prepared either by distillative separation of the excess monomer after the trimerization reaction or by stirring the trimerization reaction toward high conversions, until the monomer has very largely undergone conversion to higher-oligomer isocyanurates. The latter method is particularly successful when the diisocyanates used carry two isocyanate groups which differ in their reactivity, as in the case of 2,4-toluene diisocyanate. Solvent-containing products of this kind can be prepared in this way with a monomeric TDI (sum of the isomeric toluene diisocyanates) content of <0.5% (e.g. ®Desmodur IL, commercial product of Bayer AG, 50% in butyl acetate, NCO content: 8.0%).
As a result of the tightening in the labelling of TDI-based products, interest in substantially monomer-free grades, i.e. products having TDI contents of <0.1%, has come sharply to the fore. In order to achieve this aim, the starting base materials of TDI-based coating systems ought to contain substantially <0.5% by weight TDI, preferably <0.1% by weight.
In principle, this objective can be achieved simply by continuing trimerization to even higher conversions and thus even higher molecular weights. However, this increases the viscosity, and on the other hand this leads to products having fewer and fewer reactive isocyanate centres. Furthermore, to an increasing extent, compatibility in the case of blending with other paint base materials is unfavorably influenced in this way.
There has also been no lack of attempts to reduce the monomer content in other ways. The use of non-solvents for trimers in order to shift the equilibrium are described in, for example, JP-A 56059828. Distillation processes for “thin-filmed TDI trimers” (e.g. DE-A 3 420 923, DE-A 19 618 230) and processes which employ a pretreatment or aftertreatment stage, such as subsequent urethanization, for example, are described in the literature (DE-A 3 928 503, U.S. Pat. No. 3,384,624, DE-A 2 414 413, DE-A 19 523 657,). Even particularly gentle processes for selective trimerization at low temperatures are discussed (JP-A 63 260 915). All of the methods described, however, lead to very complex, often multistage, industrial processes, are associated with very long reaction times and hence a poor .space/time yield, and/or cannot be transferred from the laboratory stage to the industrial scale. EP-A 0 416 338 describes a catalytic trimerization of TDI in paint solvents using, inter alia, Mannich bases. The addition of alcohols, which is described here as being essential and necessary, leads, however, to the formation of urethane groups, which exert a strong co-catalytic influence on the trimerization reaction and so destroy the selectivity that is necessary for achieving low TDI contents. In analogy to this, combinations of alcohol sand other catalyst systems are described (e.g. U.S. Pat. No. 5,905,151). It was, however, not possible to obtain products whose monomer content was <0.1%.
DE-A 2 452 532 describes, inter alia, likewise the trimerization of TDI using Mannich bases. Essential to the invention in this case is the addition of carbamic esters, leading at high temperatures to the deactivation of the catalyst system and so preventing “runaway” of the reaction. However, only products with high monomer content are obtained, with no substantially monomer-free TDI trimer grades.